This invention relates to heat-sensitive recording elements particularly useful for making color hard copy, to a method of imaging employing said elements and to novel colorless precursors of preformed image dyes useful as the color image-forming materials.
Dye precursor molecules have been suggested previously which become irreversibly colored by the loss of a single group. For example, Japanese Patent Kokai No. 57-46239, Laid Open Mar. 16, 1982, discloses indoaniline dye precursors which possess an alkyl/aryl sulfonyl group that irreversibly cleaves from the precursor molecule upon exposure to light, usually ultraviolet light, with the result that the precursor is converted to its colored form and cannot revert back to its leuco or colorless form. U.S. Pat. No. 3,409,457 to Karl-Heinz Menzel discloses colorless dye precursors which possess an acylamino group that cleaves from the precursor molecule upon heating to yield a colored azomethine dye. The conversion of these leuco compounds into the azomethine dyes is accelerated by using alkalis such as alkali alcoholates. The acylamino and alkyl/aryl sulfonyl groups employed in the colorless dye precursors of these references depart from the precursor molecule to effect conjugation and form a dye chromophore.
U.S. Pat. No. 4,602,263 to Alan L. Borror, Ernest W. Ellis and Donald A. McGowan discloses the stabilization of a Colorless dye precursor by employing a tertiary-alkoxycarbonyl group, for example, t-butoxycarbonyl, as a thermally removable protecting group. This protecting group is removed by unimolecular fragmentation upon heating, which fragmentation reaction is irreversible. U.S. Pat. No. 4,720,449 to Alan L. Borror and Ernest W. Ellis discloses colorless di- and triarylmethane compounds possessing a masked acylation substituent which undergoes irreversible fragmentation upon heating to liberate the acyl group for effecting an intramolecular acylation reaction whereby the compounds are rendered colored.